The present invention relates to organic-inorganic hybrid composites which have unique and useful electronic and optical properties. More specifically, the invention relates to II-VI semiconducting chalogenides with modified structures and properties based upon the incorporation of organic components via coordination or covalent bonds.
Group II-VI semiconducting chalcogenide compounds such as CdTe and ZnSe are of great interest currently for use in semiconductor devices because of their relatively wide band gaps. Semiconductor nanostructures with uniform arrangement, such as periodic arrays of quantum dots, are necessary to achieve a sharp line width and strong intensity for practical applications in optoelectronic devices. Quantum dots grown by colloidal methods are highly attractive because of their small size and strong capability for modifying electronic and optical properties of semiconductor bulk materials. For example, InP dots with sizes ranging from two to six nanometers in diameter can shift optical gaps by as much as one electron volt. The great challenge, however, is to generate uniformly sized dots and to organize them into periodic arrays in order to obtain sharp line width, and control over intensity and other optical properties. Self-assembled strain dots have some uniform structures, but their ability to change optical properties is severally limited. This substantially restricts their uses.
There remains a need for quantum confined systems combining uniformity in structure with the ability to significantly modify the electronic and optical properties of semiconducting materials.
This need is met by the present invention. Applicants have discovered a new type of quantum confined nanostructures that are not only capable of modifying optical, electronic and other properties of a semiconductor on the same large scale as colloidal dots, but also present uniform structures that are particularly advantageous to device making. Compounds of the present invention are covalent or coordinate bonded organic-inorganic hybrid materials with a uniform, periodic nanostructure exhibiting significant quantum confinement effects.
The structures of the hybrid materials of the present invention are constructed in such a way that they contain uniformly sized II-VI semiconductor fragments as sources of the desired semiconductor functionality, and organic spacers as links or nodes to the inorganic fragment motifs in an ordered fashion. The quantum confinements induced in such systems are unusually strong, as a result of highly confined, single-atomic inorganic layers with a thickness less than one nanometer. This leads to a significant blue shift in their optical absorption edges (as high as 1.2-1.5 electron volts), compared to 1.0 electron volt shift obtained by the best-grown III-V and II-VI semiconductor quantum dots.
Therefore, according to one of the embodiment of the present invention, a quantum confined system is provided that is a crystalline organic-inorganic hybrid compound containing alternating layers of a bifunctional organic ligand and a II-VI semiconducting chalcogenide, wherein:
the semiconducting chalcogenide has the formula MQ, in which M represents one or more II-VI semiconductor cationic species and Q is a chalcogen element selected from S, Se or Te; and
the bifunctional organic ligands of each organic ligand layer are bonded by a first functional group to an element M of an adjacent II-VI semiconducting chalcogenide layer and by a second functional group to an element M from the adjacent opposing II-VI semiconducting chalcogenide layer so that the adjacent opposing II-VI semiconducting chalcogenide layers are linked by the bifunctional organic ligands of the organic ligand layers.
Among the bifunctional organic ligands, organic diamines are preferred, with organic diamines having the formula Rxe2x80x94(NH2)2 being more preferred, with R being C2-C6 straight-chained or branch, substituted or unsubstituted, saturated or unsaturated aliphatic or cycloaliphatic hydrocarbons.
For purposes of the present invention, quantum confined systems are defined as systems exhibiting electronic confinement in at least one dimension. This includes systems that function as quantum wells by exhibiting electronic confinement in one dimension, systems that function as quantum wires by exhibiting electronic confinement in two dimensions, and systems that function as quantum dots by exhibiting electronic confinement in three dimensions.
Furthermore, II-VI semiconducting chalcogenides are defined according to their well-understood meaning, in which the term chalcogenide is limited to S, Se and Te, and the semiconductor has a zinc blende or wurtzite structure. Cationic species of such semiconductor compounds include Zn, Cd, Hg and Mn.
The quantum confined systems of the present invention are prepared by a method that organizes periodic three-dimensional II-VI semiconductor host lattice segments between organic layers by way of stable coordinate or covalent bonds in an ordered manner. Because the quantum confinement effect induced in the hybrid composite materials of the present inventions is the result of inherent structural properties, the restriction on size distribution is lifted and the synthesis methods of present invention can be used to generate particles of unlimited size, with no effect upon their electronic and optical properties. This is in contrast to the properties of nanoparticles grown by colloidal methods, which depend strongly on particle size, and which are formed via uncorrelated nucleus cores, making it difficult to generate particles with the requisite narrow size distribution and ordered structure.
Therefore, according to another aspect of the present invention, a method is provided for the preparation of the crystalline, covalent or coordinate bonded, organic-inorganic hybrid chalcogenide quantum confined systems of the present invention, in which a mixture is heated containing:
(a) a salt of a II-VI semiconductor cationic species; (b) a chalcogen selected from S, Se and Te; and (c) a bifunctional organic compound capable of covalent or coordinate bonding with the cationic species;
at a temperature effective to form the hybrid chalcogenide, until the hybrid chalcogenide is formed.
The alternating semiconductor and organic layers of the hybrid material of the present invention, prepared by the methods described herein, mimic a superlattice structure. However, unlike the conventional semiconductor superlattices where the band offset introduces only a weak confinement, the insulating organic layer will impose a strong confinement on the semiconductor layer, giving rise to a large variation with respect to the bulk semiconductor properties. In addition, the hybrid organic-inorganic nature of the composites of the present invention provides advantages, features and properties that are important for the miniaturization of electronic and optical devices. Representative features include superior electronic and optical properties, as well as rigidity and stability provided by the inorganic component, and high processibility, flexibility, weight reduction and structural diversity provided by the organic component. Therefore, according to another aspect of the present invention, a semiconductor device is provided containing multiple layers of the crystalline organic-inorganic hybrid compounds of the present invention. The semiconductor devices of the present invention are fabricated by known techniques.
As can be appreciated by one skilled in the art, variation of the II-VI semiconducting chalcogenide and bifunctional organic compounds will provide a broad range of hybrid compounds exhibiting a wide range of properties